Interpretive Summary: Biochar amendment to temperate soils is being suggested as a strategy to improve soil fertility and mitigate climate change, but before this can become a recommended management practice, a better understanding of the impacts of biochar on the soil biota is needed. We determined the effect of addition rates (0, 1, 5, 10, and 20% by weight) of a fast-pyrolysis wood-derived biochar on the abundance and temporal dynamics of microbial community biomarkers (phosopholipid fatty acids, PLFAs) in four temperate soils during one year incubation. Though chemically recalcitrant, our biochar served as a substrate for microbial activity, more so at high addition rates and in soil with little organic matter. Biochar addition also increased microbial abundance in all soils and proportionally to addition rates, but did not alter the community composition. Microbial use of biochar-C could only partially explain this observed benefit, suggesting that other mechanisms were involved. Overall our study provides support for the beneficial use of biochar as a soil amendment at high addition rates by alleviating stresses on the microbial community.

Technical Abstract:
Biochar amendment to temperate soils is being suggested as a strategy to improve soil fertility and mitigate climate change. Yet, before this can become a recommended management practice, a better understanding of the impacts of biochar on the soil biota is needed. We determined the effect of addition rates (0, 1, 5, 10, and 20% by weight) of a fast-pyrolysis wood-derived biochar on the abundance and temporal dynamics of phospholipid fatty acids (PLFAs, a microbial community biomarker) in four temperate soils during one year incubation. Additionally, the effects on microbial mineralization/incorporation of biochar-C were determined by following through time the production of CO2 and the biochar contribution to CO2 and PLFA-C using the natural 13C abundance differences between biochar and soils. Though chemically recalcitrant, our biochar served as a substrate for microbial activity, more so at high addition rates and in soil with little organic matter. Biochar addition also exerted a benefit on microbial abundance in all soils and proportionally to addition rates, but did not alter the community composition. Microbial use of biochar-C could only partially explain this observed benefit, suggesting that other mechanisms were involved. Finally, we also observed a strong decrease in the PLFA extraction efficiency in all soils upon biochar addition. These findings emphasize the need to measure and correct for extraction efficiencies to correctly investigate soil microbial responses to biochar additions when using PLFA biomarker. Overall our study provides support for the beneficial use of biochar as a soil amendment at high addition rates by alleviating stresses on the microbial community.